The present invention relates to electrical oils. More particularly the present invention relates to electrical oils derived from paraffinic or naphthenic distillates that have been treated to be substantially sulfur free and from a hydrofined light gas oil.
Generally, electrical oils are prepared from naphthenic crude oils by a variety of processes. In one process, a vacuum distillate of a naphthenic crude is solvent extracted with phenol to remove polycondensed ring aromatics, nitrogen and sulfur compounds and then is hydrofined to impart good color, odor and oxidation stability and electrical properties. Electrical oils produced in this manner normally meet or exceed requisite performance conditions exemplified by ASTM D 3487 and ASTM D 2440.
In another process, a vacuum distillate is hydrotreated under conditions which increase saturates and removes nitrogen compounds and up to about 90% of sulfur compounds. To meet the oxidation requirements of an electrical oil, a synthetic oxidation inhibitor, such as 2, 6 di-t-butyl phenol or 2, 6 di-t-butyl cresol then is added to the thus hydrotreated distillate.
It now has been discovered that an electrical oil having excellent oxidation stability, impulse breakdown strength and gassing tendency in the absence of added oxidation inhibitor is obtained by blending a substantially sulfur free paraffinic or naphthenic base oil boiling in the electrical oil range with a hydrofined light gas oil having a sulfur to basic nitrogen ratio greater than 100:1, the amount blended being an amount sufficient to provide a blend having a greater than about 0.03 wt % sulfur.
In preparing the blended electrical oil of the present invention a substantially nitrogen and sulfur free base oil obtained by treating a paraffinic or naphthenic distillate boiling in the electrical oil range, for example in the range of 225xc2x0 C. to 480xc2x0 C. at atmospheric pressure, is employed. Typically such base oils will have less than about 500 ppm sulfur, for example 50 to 300 ppm and less than about 25 ppm basic nitrogen, i.e., between about 1 to 5 ppm. Examples of such base oils are those that have been treated or obtained from distillates that have been treated under conditions that substantially lower the nitrogen and sulfur compounds present in the base oil or distillate and increase the saturates present to greater than 75 wt % as determined by clay gel . Thus in one embodiment, a suitable paraffinic or naphthenic distillate is obtained by distilling a crude oil feedstock. The resultant distillate is then treated with an aromatic selective solvent such as phenol, N-methyl pyrolidone, or furfural, to remove aromatic compounds and to decrease the amount of nitrogen and sulfur compounds present. Such solvent extraction is well known. Typical extracting temperatures are in the range of 50xc2x0 C. to 100xc2x0 C. and the volume ratios of solvent to distillate in the range of 1:1 to 2:1.
The solvent extracted distillate is next hydrofined under known hydrofining conditions to lower the basic nitrogen levels in the distillate to less than 25 ppm, typically below about 10 ppm and preferably between about 1 ppm to about 5 ppm. As is well known, basic nitrogen compounds are those that can be titrated with perchloric acid using acetic acid as a solvent in contrast to other nitrogen compounds present in the oil which are not titratable. Typical hydrofining conditions for the solvent extracted distillate are given in Table 1.
The substantially sulfur free treated distillate is then blended with a light gas oil (LGO). Typically the LGO is one boiling in the range of about 200xc2x0 C. to 400xc2x0 C. at atmospheric pressure, i.e., the LGO distillate employed is one having a minimum flash point of 140xc2x0 C., preferably greater than 145xc2x0 C. and a viscosity of about 40 SUS@ 100xc2x0 F.
The LGO distillate preferably is one that has been hydrofined to improve color and odor and reduce the basic nitrogen level, while maintaining a sulfur (S) to basic nitrogen (BN) weight ratio of greater than 100:1 and preferably greater than about 200:1. Typical conditions for carrying out this hydrofining are shown in Table 2.
The resultant hydrofined LGO is added to the solvent refined distillate in an amount sufficient to provide an electrical oil having greater than 0.03 wt % sulfur, for example between 0.03 wt % to 1 wt % and preferably from about .05 wt % to about 0.2 wt %. Typically the volume ratio of solvent extracted and hydrofined distillate to hydrofined LGO will be in the range of about 75:25 to about 25:75.
In order to obtain an electrical oil having a desired pour point, a pour point depressant such as an alkylated polystyrene may be added to the blended composition. Alternatively, the solvent extracted and hydrofined distillate may be subjected to solvent or catalytic dewaxing before blending with the LGO. In yet another embodiment the blended composition may be subjected to solvent or catalytic dewaxing.
The oxidation stability of the composition of the present invention can be even further enhanced by the addition of a minor but effective amount of an oxidation inhibitor such as 2,6 di-t-butyl phenol and 2,6 di-t-butyl cresol. Thus for a Type I electrical oil less than 0.08 wt % of inhibitor may be added and for a Type II oil less than about 0.3 wt %.